Hydrostatically steered vehicles have been available for many years and their application to windrowers has been significant. There has been a limit to the transport speed of these windrowers due to instability at high speed with the drive wheels ahead of the castor wheels (cab forward). Higher speeds on the road allow the user to cover large distances without the use of trailers and tow vehicles.
Commonly used hydrostatic propulsion system for a tractor provides variable flow output by step-less or continuous alteration of pump displacement where the pump is driven by the engine at a constant speed. The motor which receives the flow from the pump can also be varied in displacement to alternate between high torque/low speed or low torque/high speed modes. The motor displacement control is typically implemented in discrete steps or ranges.
Transitioning the motor between high torque/low speed and low torque/high speed ranges while the vehicle is in motion is a desirable transmission characteristic. From a standstill and at lower speeds, high torque permits overcoming stopped vehicle inertia, rolling resistance, effort to ascend grade. When lower torque requirements exist the vehicle speed can be increased to reduce transit time or improve system efficiency by reducing the speed of the power source driving the pump.
In a dual path hydrostatic system, such as a windrower vehicle there are two independent hydrostatic systems which drive the wheels on opposite sides of the machine. By varying each wheel rotation the vehicle speed, direction, and steering is accomplished. The typical application uses a compact speed reduction gearbox that fits inside and supports the wheel. This gearbox is driven by the motor and the torque is multiplied to the level suitable for vehicle propulsion. On a dual path vehicle the gearbox ratio is typically fixed so the variable motor must provide a wide range of output speeds to accommodate high torque/low speed working operations and low torque/high speed transportation between working locations. The gearbox ratio is fixed as synchronizing a gearbox ratio shifting operation between two independent gearboxes would be difficult to maintain commanded wheel speeds and importantly steering control. Synchronizing a fixed displacement motor speed of each individual hydrostatic system to permit a mechanical ratio change of gears would be equally complex and difficult. A pair of infinitely variable ratio type gearboxes could be applied in place of or in series with a fixed gearbox, however it is easily realized this reduces the economy in application due to the complex nature of this type of mechanism.
In service braking on a dual path vehicle it is difficult to utilize common friction braking techniques as it would be complex to accurately apply the correct amount of braking torque on independently braked wheels to limit the effects on the steering control.
Motor displacement controls that shift between 2 or 3 discrete steps may rely on internal springs and forces applied by hydraulic pressure and uncontrolled flow which provides unpredictable shifting characteristics, especially to dual path applications where the hydraulic pressure is supplied in parallel to both motors. In dual path application, during motor shifting this can result in undesirable steering deviation. Discrete shifting steps also limit the flexibility to accommodate within a fixed range an infinite speed/torque setting.